Chap, vi.] GALVANIC KEYS. 55 



wire can be inserted into it and fixed with a screw. 



A second wire with a similar projecting end is movable 



round an axis by an insulated handle. 



It can be caused to dip into the 



mercury or can be taken out. Wires 



from the battery being connected with 



both the binding screws, the current is 



fi(r 29 JVIer- 



passed when both wires dip into the e cury key. 

 mercury and is interrupted when the 

 handle is pulled backwards so as to lift one wire out. 

 A contact or spring 1 key is shown in Fig- 30, 

 On a vulcanite or other insulating block is fixed a 

 spring, in metallic connection with a binding screw at 

 the base of the pillar supporting it. The end of the 

 spring has a little ivory button, 3, by which the finger 

 may press the spring down so as to 

 make contact with the metal pillar 1, 

 which is in metallic connection with 

 the binding screw 2. Thus, the elec- 



Fig. 30. Spring , i /> i j ji i i 



Key. trodes being fixed to the binding screws, 



the current can pass only when the 

 spring is pressed down by the finger to make contact 

 with the pillar 1. 



The friction key of Du Bois-Reymoiid is repre- 

 sented in Fig. 31. It consists of a plate of vulcanite 

 G attached to a screw clamp for fixing it to the edge 

 of a table. On the plate are two rectangular pieces 

 of brass A and B, placed in the position shown in the 

 figure. Each piece of brass has two holes drilled 

 through it, and a screw passes down to each hole to fix 

 any wire that may be inserted. A bridge of brass is 

 pivoted to B in such a way that when lowered by the 

 insulating handle c it makes close contact with the 

 end section of A. There are two ways of interposing 

 this key in a circuit. The best way is that shown in 

 Fig. 31, viz. : Carry a wire from the positive pole of 

 the element to binding screw 1, and from 2 on the same 



